infection

infection,

invasion of plant or animal tissues by microorganisms, i.e., bacteriabacteria[pl. of bacterium], microscopic unicellular prokaryotic organisms characterized by the lack of a membrane-bound nucleus and membrane-bound organelles. Once considered a part of the plant kingdom, bacteria were eventually placed in a separate kingdom, Monera......Click the link for more information., virusesvirus,parasite with a noncellular structure composed mainly of nucleic acid within a protein coat. Most viruses are too small (100–2,000 Angstrom units) to be seen with the light microscope and thus must be studied by electron microscopes......Click the link for more information., viroidsviroid,microscopic infectious agent, much smaller than a virus, that infects higher plants such as potatoes, tomatoes, chrysanthemums, and cucumbers, causing stunted or distorted growth and sometimes death. It can be transmitted by pollen, seed, or farm implements......Click the link for more information., fungiFungi, kingdom of heterotrophic single-celled, multinucleated, or multicellular organisms, including yeasts, molds, and mushrooms. The organisms live as parasites, symbionts, or saprobes (see saprophyte)......Click the link for more information., rickettsiasrickettsia, any of a group of very small microorganisms, many disease-causing, that live in vertebrates and are transmitted by bloodsucking parasitic arthropods such as fleas, lice (see louse), and ticks......Click the link for more information., and protozoansprotozoan, informal term for the unicellular heterotrophs of the kingdom Protista. Protozoans comprise a large, diverse assortment of microscopic or near-microscopic organisms that live as single cells or in simple colonies and that show no differentiation into tissues......Click the link for more information.. The invasion of body tissues by parasitic wormsworm,common name for various unrelated invertebrate animals with soft, often long and slender bodies. Members of the phylum Platyhelminthes, or the flatworms, are the most primitive; they are generally small and flat-bodied and include the free-living planarians (of the class.....Click the link for more information. and other higher organisms is commonly referred to as infestation.

Invading organisms such as bacteria produce toxinstoxin,poison produced by living organisms. Toxins are classified as either exotoxins or endotoxins. Exotoxins are a diverse group of soluble proteins released into the surrounding tissue by living bacterial cells. Exotoxins have specific reaction sites in the host; e.g......Click the link for more information. that damage host tissues and interfere with normal metabolism; some toxins are actually enzymes that, by breaking down host tissues, prevent the localization of infections. Other bacterial substances destroy the host's phagocytes. Viruses and retrovirusesretrovirus,type of RNA virus that, unlike other RNA viruses, reproduces by transcribing itself into DNA. An enzyme called reverse transcriptase allows a retrovirus's RNA to act as the template for this RNA-to-DNA transcription......Click the link for more information. are parasitic on host cells, causing cellular degeneration, as in rabiesrabiesor hydrophobia, acute viral infection of the central nervous system in dogs, foxes, raccoons, skunks, bats, and other animals, and in humans. The virus is transmitted from an animal to a person, or from one animal to another, via infected saliva, most often by.....Click the link for more information., poliomyelitispoliomyelitis, polio,or infantile paralysis,acute viral infection, mainly of children but also affecting older persons. There are three immunologic types of poliomyelitis virus, one of which was eradicated in 1999; exposure to one type produces immunity.....Click the link for more information., and AIDSAIDSor acquired immunodeficiency syndrome,fatal disease caused by a rapidly mutating retrovirus that attacks the immune system and leaves the victim vulnerable to infections, malignancies, and neurological disorders. It was first recognized as a disease in 1981......Click the link for more information., or cellular proliferation, as in wartswart,circumscribed outgrowth of the skin caused by a filterable virus that is readily transmitted. Warts may appear anywhere on the skin but are most common on the hands......Click the link for more information. and cold sores. Some viruses have been associated with the development of certain cancerscancer,in medicine, common term for neoplasms, or tumors, that are malignant. Like benign tumors, malignant tumors do not respond to body mechanisms that limit cell growth......Click the link for more information.. Substances produced by many invading organisms cause allergic sensitivity in the host; the immune response to virus infection has been implicated in some diseases (see allergyallergy,hypersensitive reaction of the body tissues of certain individuals to certain substances that, in similar amounts and circumstances, are innocuous to other persons. Allergens, or allergy-causing substances, can be airborne substances (e.g......Click the link for more information.).

Infections may be spread via respiratory droplets, direct contact, contaminated food, or vectors, such as insects. They can also be transmitted sexually (see sexually transmitted diseasessexually transmitted disease(STD) or venereal disease,term for infections acquired mainly through sexual contact. Five diseases were traditionally known as venereal diseases: gonorrhea, syphilis, and the less common granuloma inguinale, lymphogranuloma venereum, and.....Click the link for more information.) and from mother to fetus. Immunityimmunity,ability of an organism to resist disease by identifying and destroying foreign substances or organisms. Although all animals have some immune capabilities, little is known about nonmammalian immunity......Click the link for more information. is the term used to describe the capacity of the host to respond to infection. Drugs that help fight infections include antibioticsantibiotic,any of a variety of substances, usually obtained from microorganisms, that inhibit the growth of or destroy certain other microorganisms. Types of Antibiotics.....Click the link for more information. and antiviral drugsantiviral drug,any of several drugs used to treat viral infections. The drugs act by interfering with a virus's ability to enter a host cell and replicate itself with the host cell's DNA......Click the link for more information..

See also specific diseases, diseases of plantsdiseases of plants.Most plant diseases are caused by fungi, bacteria, and viruses. Although the term disease is usually used only for the destruction of live plants, the action of dry rot and the rotting of harvested crops in storage or transport is similar to the rots.....Click the link for more information..

Bibliography

See J. Waller, The Discovery of the Germ (2003).

Infection

A term considered by some to mean the entrance, growth, and multiplication of a microorganism (pathogen) in the body of a host, resulting in the establishment of a disease process. Others define infection as the presence of a microorganism in host tissues whether or not it evolves into detectable pathologic effects. The host may be a bacterium, plant, animal, or human being, and the infecting agent may be viral, rickettsial, bacterial, fungal, or protozoan.

Infection

penetration of a pathogenic parasite into a human or animal organism and the state of being infected. The concept of infection is also applied to one-celled organisms (bacteriophages). On the other hand, there is a tendency to distinguish between the concept of infection and that of parasitism, including invasion.

Upon entering the body, the causative agent concentrates in certain organs and tissues. For example, in the course of evolution the itch mite adapted to parasitizing the epithelial layer of skin; the causative agent of typhus, Rickettsia prowazekii, the wall of arterioles and arterial capillaries; and the influenza virus, the mucous membrane of the upper respiratory tract. After the parasite enters the body, a complex interaction takes place between the parasite and host—an infectious process. The infectious process (dynamics of pathological changes) includes the causative agent’s adaptation to new conditions of existence and its reproduction, dissemination of the process, metastasis, and intoxication of the host. The infectious process and the functional disturbances in the host and its reflex reactions constitute the pathogenetic essence of infectious disease.

An infection is manifested as an acute or chronic form of the disease or carrier state. The development of a particular form depends, on the one hand, on the properties of the causative agent—its infectiousness, invasiveness, and capacity to form exotoxins and endotoxins—and on the number of parasites entering the organism. On the other hand, the condition of the organism and degree of susceptibility or predisposition to a given disease is a very important factor. The presence of certain causative agents in the organism does not provoke a pathological process unless some other conditions exist. Such causative agents are said to be conditionally pathogenic. They include the large group of causative agents of wound infections, many Escherichiaspecies, and herpesvirus. They may exist a long time without producing symptoms on the skin, on the mucous membranes, and in the intestine until an injury, chill, or other factor enables them to manifest their pathogenic properties.

An infected organism or carriers of the causative agents are the sources of infections. Every infectious disease has its own specific mechanism of transmission. In intestinal infections, such as dysentery or typhoid fever, the causative agent is eliminated from the body with feces or urine and in one way or another enters the mouth of a healthy individual. In infectious diseases of the respiratory tract, the causative agent is eliminated with drops of mucus during sneezing, coughing, or talking and penetrates into a healthy individual with inhaled air (the droplet mechanism of transmission). In typhus, malaria, bubonic plague, and some other diseases, the causative agent is trasmitted by blood-sucking insects—lice, mosquitoes, and fleas—parasitizing first a sick and then a healthy individual. The causative agents of scabies and fungal and venereal diseases are transmitted by direct contact with a diseased individual.

A knowledge of the mechanisms of transmission of infection is the basis of prevention of infectious diseases.

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